Various pulse wave radar devices, which measure a distance to an object based on time while a pulse wave is transmitted and the pulse wave reflected from the object is received, are known. In such pulse wave radar devices, the time from transmission of a modulated pulse wave to reception of a received wave is measured so that a signal which is proportional to a distance between the pulse wave radar devices and the objects is obtained, and the distance is calculated based on the signal.
The pulse radar devices are utilized as position/distance measuring devices, which are mounted to meteorology surveys and airplanes whose distance with respect to an object is about 100 km. In order to enhance the sensitivity in such applications, a modulated pulse wave with pulse width of about several hundred nsec is used. When, however, the modulated pulse wave with pulse width of about several hundred nsec is used even in the case where an object is at short range of about 10 cm, a reflected wave from the object is received within time of the pulse width for which a transmitting antenna transmits the modulated pulse wave, and thus a wave cannot be received normally.
For this reason, the pulse width of the modulated pulse wave is required to be 1 nsec or less, but it is difficult that a conventionally used PIN diode constitutes a pulse modulator which treats such a modulated pulse wave with narrow pulse width. As a conventional technique, a pulse modulator which uses a Schottky barrier diode or FET is proposed (for example, see Patent Document 1). The pulse width of the pulse modulator having such a configuration depends on a pulse width of a pulse generated from a pulse generating circuit.
In order to enable only the pulse generating circuit to generate a pulse with narrow pulse width, the pulse generating circuit has to be composed of elements, which operate at high speed. The elements, which operate at high speed are expensive and cause an increase in power consumption.
On the other hand, the pulse wave radar device measures time from the transmission of a transmitting wave to the reception of a receiving wave so as to obtain a signal, which is proportional to a distance from the pulse wave radar device to an object, and calculates a distance to the object from the signal. Actually, since the pulse from the pulse generating circuit is used as a start signal, an internal propagation delay of a pulse modulator or the like is estimated so that the measured time is corrected. When the modulated pulse wave with narrow pulse width is treated, however, even a slight error in internal propagation delay greatly influences an error in the measured time.
[Patent Document 1 ]
Japanese Patent Application Laid-Open No. 2000-258525